The present invention relates to a charge-transfer modulator-demodulator.
It will be recalled that a modulator-demodulator, currently called modem, comprises:
a modulator which converts a random succession of binary signals equal to 0 or 1 into a periodic analog signal, modulated in frequency, in amplitude or in phase, and transmissible by means of the telephone network;
a demodulator which converts the modulated analog signal supplied by the network into a succession of 0 and 1.
The present invention relates to frequency-modulation modems which are known under the English name of Frequency Shift Keying or FSK.
In FSK modems, the modulator causes a sine curve at frequency F.sub.A to correspond to a zero and the modulator causes a sine curve F.sub.Z to correspond to a one.
The present invention relates to asynchronous FSK modems, i.e. in which passing over from the frequency F.sub.A to the frequency F.sub.Z and vice versa takes place at any moment in the period of the analog signal.
The specifications of the CCITT lay down all the characteristics of modems, thus a 1200-baud modem has been studied for which F.sub.A =2100 Hz and F.sub.Z =1300 Hz. This modem will be cited by way of example in what follows.
In asynchronous FSK modems, the linking up of the sine curves at frequencies F.sub.A and F.sub.Z must take place perfectly whatever the moment when it occurs.
For that, modulators are known in the prior art which only comprise a single oscillator at a multiple frequency of F.sub.A and F.sub.Z. Thus, in the case where F.sub.A =2100 Hz and F.sub.Z =1300 Hz, the oscillator delivers 100.times.13.times.21 Hz. The signal delivered by the oscillator is chopped then fed into a logic frequency divider which enables 1300 or 2100 Hz to be produced with a perfect linking up of the analog signals supplied.
To further minimize the parasites caused by the frequency change, an oscillator is used producing (100.times.13.times.21) Hz, where k is of the order of ten. The linking then takes place between analog signals at frequencies k.1300 Hz and k.2100 Hz. These signals are then fed to a programmable function generator which comprises k successive states and which enables a substantially sinusoidal 1300 or 2100 Hz sampled signal to be recovered.
There is also known from French patent application No. 78.19141 published under the No. 2.414.835 in the name of Texas Instruments, a modulator for an FSK modem (FIGS. 17 and 18), which comprises a programmable function generator formed from capacitors C.sub.A to C.sub.G switched by MOS transistors connected as inverters INV 1 to INV 7. The inverter-transistors are controlled by exciter MOS transistors S.sub.1 to S.sub.7 connected to a shift register.
By controlling the shift register, a sampled signal is synthesized which is substantially sinusoidal at two distinct frequencies F.sub.A and F.sub.Z.
The modulators for FSK asynchronous modems of the prior art present the disadvantage of being integrable only with difficulty for they comprise either a quartz oscillator or numerous capacitors of high values. These modulators also present the disadvantage of being costly to produce.